RuntimeDyldELF: report_fatal_error instead of asserting for unimplemented relocations (PR46816)

This fixes the ExecutionEngine/MCJIT/stubs-sm-pic.ll test in no-asserts
builds which is set to XFAIL on some platforms like 32-bit x86. More
importantly, we probably don't want to silently error in these cases.

Differential revision: https://reviews.llvm.org/D84390